The present invention relates to a polyamide which has a high polarity and a high refractive index and which is useful as a material for producing fibers or plastics; to a process for producing the polyamide; and to an intermediate for producing the polyamide.
Examples of materials and products prepared from polyamides includes polyamide fiber such as Nylon, polyamide resin employed as engineering plastics, and aramid fiber such as Kevlar fiber. These polyamides are widely employed in such applications as general purpose fiber materials, elastic fiber materials, high-strength fiber materials, and plastics.
However, there is a demand for the development of a polyamide that exhibits enhanced properties. One example is swimwear. When white swimwear fabricated with a conventional nylon is worn, the body of the wearer can be seen through the swimwear due to the proximity of the refractive index of water and that of nylon. Meanwhile, swimwear fabricated with a conventional high-refractive-index polyamide is disadvantaged by feeling uncomfortable against the skin and by having an unsatisfactory appearance. Thus, there is a demand for a polyamide that can provide a fibersatisfying both a high refractive index and having a good sensation against the skin. In addition, conventional polyamide has poor biodegradability, and thus a polyamide having improved biodegradability is demanded.
In view of the foregoing, an object of the present invention is to provide a polyamide having functions such as a high refractive index, high strength, and biodegradability.
In order to overcome the aforementioned drawbacks, the present inventors have conducted extensive studies on dicarboxylic acids which can serve as raw materials for producing polyamides, and have found that polyamides which are produced by polycondensing 2H-pyran-2-one-4,6-dicarboxylic acid and a variety of diamines exhibit high mechanical strength and have a high refractive index and polarity and that a 2H-pyran-2-one ring has excellent biodegradability. The present invention has been accomplished on the basis of these findings.
Accordingly, the present invention provides a polyamide having a structural repeating unit represented by formula (1) 
(wherein R1 represents a divalent hydrocarbon residue optionally having in the structure a heteroatom having no active hydrogen), and a process for producing the same.
The present invention also provides a polyamide having structural repeating units represented by formula (1) and (2): 
(wherein R2 represents a divalent hydrocarbon residue optionally having in the structure a heteroatom having no active hydrogen and R1 represents the same as defined above), and a process for producing the same.
The present invention further provides 2H-pyran-2-one-4,6-dicarboxylic acid derivatives represented by formula (3): 
(wherein X1 represents an alkoxy group or a halogen atom). Among the monomers for producing the polyamide of the present invention, the above carboxylic acid derivatives are novel compounds.
The polyamides of the present invention have a structural repeating unit represented by formula (1) or structural repeating units represented by formulas (1) and (2). In formulas (1) and (2), each of R1 and R2 represents a divalent hydrocarbon residue optionally having in the structure a heteroatom having no active hydrogen. Among divalent hydrocarbon residues, preferred are R3 and R3xe2x80x94(OR3)1xe2x80x94, wherein 1 is a number between 1 and 4; and R3 represents a C2-C24 saturated or unsaturated divalent hydrocarbon residue, with a C2-C24 linear chain or branched chain alkylene group, a C3-C8 cycloalkane divalent residue, and a C5-C10 aromatic hydrocarbon divalent residue being particularly preferred. Examples of more preferred R1 and R2 include a trimethylene group, a propylene group, a tetramethylene group, a hexamethylene group, an octamethylene group, a decamethylene group, a dodecamethylene group, a phenylene group, a tolylene group, a xylylene group, a naphthalene group, a cyclohexylene group, and xe2x80x94CH2CH2(OCH2CH2)2xe2x80x94. These hydrocarbon residues may have a substituent having no active hydrogen such as an alkoxy (preferably C1-C6) group, an alkanoyl (preferably C2-C6) group, an alkyl (preferably C1-C6) group, an aryl (preferably C6-C14) group, or an aralkyl (preferably C7-C18) group.
In the polyamide having structural repeating units represented by formulas (1) and (2), these two types of units may be linked in a block manner (i.e., block copolymer) or randomly (i.e., random copolymer).
No particular limitation is imposed on the molecular weight of the polyamide of the present invention, and it varies in accordance with use. Typically, the molecular weight based on the number average molecular weight is preferably 10,000-200,000, more preferably 40,000-100,000. The molecular weight is particularly preferably 60,000-80,000 in view of moldability from a solution or melt thereof and development of physical properties such as mechanical strength.
The polyamide of the present invention having a structural repeating unit represented by formula (1) may be produced in accordance with the following reaction scheme: 
(wherein X2 represents a hydroxyl group, an alkoxy group, or a halogen atom; R1 is the same as defined above).
Specifically, a 2H-pyran-2-one-4,6-dicarboxylic acid derivative (4) and a diamine (5) are subjected to polycondensation reaction, to thereby produce the polyamide of the present invention (1).
Among dicarboxylic monomers (4), a monomer in which X2 is an alkoxy group and a monomer in which X2 is a halogen atom are novel compounds. The compounds can be produced by converting 2H-pyran-2-one-4,6-dicarboxylic acid represented by formula (4) (wherein X is OH) to an ester or an acid halide thereof through a customary method. Among alkoxy groups for X2, a lower alkoxy group is preferred, with a C1-C6 alkoxy group being particularly preferred in view of reactivity to diamines. Among halogen atoms, a chlorine atom and a bromine atom are preferred.
Appropriate types of polycondensation may be employed in accordance with dicarboxylic species (4). For example, dehydration-polycondensation is preferably employed when a dicarboxylic species (4) in which X2 is a hydroxyl group is used. Similarly, alcohol-removing-polycondensation is preferably employed when a dicarboxylic derivative (4) in which x2 is an alkoxyl group is used and interfacial-polycondensation is preferably employed when a dicarboxylic derivative (4) in which X2 is a halogen atom is used.
In dehydration-polycondensation, for example, dicarboxylic acid (4) and a diamine (5) are mixed at a mol ratio of approximately 1:1, and the mixture is heated in the presence of an optional dehydration-condensing agent such as dicyclohexylcarbodiimide.
In alcohol-removing-polycondensation, for example, a dicarboxylic acid diester (4) and a diamine (5) are mixed at a mol ratio of approximately 1:1, and the mixture is heated.
Interfacial-polycondensation is carried out, for example, by employing an interface between a solution of a dicarboxylic dihalide (4) in halohydrocarbon and an aqueous solution containing alkali hydroxide and a diamine (5). Examples of halohydrocarbon include tetrachlorocarbon and chloroform. Sodium hydroxide is preferably used as the alkali hydroxide.
The polyamide of the present invention having structural repeating units represented by formulas (1) and (2) may be produced in accordance with the following reaction scheme: 
(wherein X3 represents a hydroxyl group, an alkoxy group, or a halogen atom; m and n are integers; and R1, R2, and X2 are the same as defined above).
Specifically, a 2H-pyran-2-one-4,6-dicarboxylic acid derivative (4), a diamine (5), and a dicarboxylic acid derivative (6) are subjected to polycondensation reaction, to thereby produce the polyamide of the present invention having structural repeating units represented by formulas (1) and (2).
Among alkoxy groups for X3 in the dicarboxylic monomer (6), a lower alkoxy group is preferred, with a C1-C6 alkoxy group being particularly preferred in view of reactivity to diamines. Among halogen atoms, a chlorine atom and a bromine atom are preferred.
Appropriate types of polycondensation may be employed in accordance with dicarboxylic species (4) and (6). For example, dehydration-polycondensation is preferably employed when dicarboxylic species (4) and (6) in which each of X2 and X3 is a hydroxyl group are used. Similarly, alcohol-removing-polycondensation is preferably employed when dicarboxylic species (4) and (6) in which each of X2 and X3 is an alkoxyl group are used, and interfacial-polycondensation is preferably employed when dicarboxylic species (4) and (6) in which each of X2 and X3 is a halogen atom are used.
These reactions may be carried out in a manner similar to that employed in the aforementioned reaction scheme 1, except that an arbitrary amount of dicarboxylic acid derivative (6) is added.
The ratio of structural repeating units (1) to (2) (m:n) is preferably 4:1-1:4, with 2:1-1:2 being particularly preferred.
A variety of additives may optionally be added to the polyamide of the present invention. Examples of additives include an anti-oxidant, a colorant, a UV-absorber, a light stabilizer, a silane coupling agent, a storage stabilizer, a plasticizer, a lubricant, a solvent, a filler, an anti-aging agent, a wettability-modifier, and a coatability-modifier.
In the thus-obtained polyamides of the present invention, a 2H-pyran-2-one ring structure imparts rigidity to the polyamides. The polyamides may have a repeating unit combination of xe2x80x9crigid ring unit-soft aliphatic chain unitxe2x80x9d or that of xe2x80x9crigid ring unit-rigid aromatic group unitxe2x80x9d by selecting R1 and R2. The polyamides having such a structure serve as materials having a wide range of physical properties and are useful as soft, elastic, or high-strength fibers and plastics. In addition, since the 2H-pyran-2-one ring has a high polarity and refractive index, the polyamides of the present invention obtained therefrom also have a high polarity and refractive index and are applicable to fiber for cloth. Furthermore, the 2H-pyran-2-one ring is found in an intermediate product during biodegradation of lignin by wild-type bacteria and is further degraded in soil to produce carbon dioxide and water. Thus, the polyamides of the present invention undergo rapid biodegradation by lignin-degrading wild-type bacteria in soil.